1. Field of the Invention
This invention relates to an electrophotographic apparatus.
2. Description of the Related Art
Image forming apparatus of electrophotographic systems, i.e., electrophotographic apparatus, have electrophotographic photosensitive members on which electrostatic latent images and toner images are to be formed. Such electrophotographic photosensitive members are required to have quality and stability for electrophotographic performance (electric-potential characteristics such as charging performance, photosensitivity and residual potential and image characteristics such as resolution and gradation) and durability (wear resistance, print resistance, environmental resistance and chemical resistance. Those having a conductive substrate and provided thereon in layers a photoconductive layer as a photosensitive layer and a surface layer are proposed as the electrophotographic photosensitive member. The electrophotographic photosensitive member is hereinafter also simply termed “photosensitive member”.
For the surface layer, various materials and layer constitution have been proposed until now. As an example thereof, they may include amorphous silicon materials which contain silicon atoms (hereinafter also termed “Si” or “Si atoms”). In particular, amorphous silicon carbide (hereinafter also termed “a-SiC”) incorporated with carbon atoms (hereinafter also termed “C” or “C atoms”) in addition to Si atoms has been put into practical use. As the a-SiC, an a-SiC incorporated with an element other than the C is also proposed. For example, it may include amorphous hydrogenated silicon carbide (a-SiC:H) incorporated with C atoms and hydrogen atoms (hereinafter also termed “H” or “H atoms”) and amorphous fluorinated silicon carbide (a-SiC:F) incorporated with C atoms and fluorine atoms. These are hereinafter generally referred to as “amorphous silicon carbide” and also termed the “a-SiC”. The surface layer formed of such a C-containing amorphous silicon material has superior electrical properties and optical properties, contributes to superior image characteristics and also has superior durability standing on a high hardness.
As the electrophotographic photosensitive members, those having such a surface layer and a photoconductive layer of amorphous silicon type in combination (the amorphous silicon is hereinafter also termed “a-Si”) have already been put into practical use.
In the electrophotographic apparatus, in order to uniformly charge the photosensitive member electrostatically by means of a charging assembly, corona discharging is often used as a charging system. However, discharge products as exemplified by oxygen ions, ozone, nitrogen oxides and various radicals are produced by this corona discharging. Then, these discharge products stay inside the electrophotographic apparatus at a high concentration, whereupon such discharge products may make the surface of the photosensitive member change in properties to change the quantity of wear of the surface layer.
As the cause of a change in the quantity of wear of the photosensitive member surface, it is considered that the discharge products make the surface layer change in properties into a film having a low hardness. As another factor, it is considered that the surface layer changes in properties to make a cleaning blade change in state of its rubbing against the photosensitive member surface.
Such changes in properties of the surface layer that are caused by the discharge products may also progress non-uniformly in the direction of rotational axis. This is, as one of the causes, due to the fact that the discharge products differ in their concentration at places inside the electrophotographic apparatus, in particular, at places around the photosensitive member. The difference in concentration of discharge products that is caused at places around the photosensitive member is greatly concerned with influences of air current which are due to the draw-in and draw-out of air that is caused by a fan provided inside the electrophotographic apparatus. Then, any non-uniform progress of such changes in properties of the surface layer finally causes the phenomenon that the surface layer differs in its quantity of wear in the direction of rotational axis.
Such discharge products (such as ozone and nitrogen oxides) are produced also in charging systems other than the corona discharging, e.g., in a roller charging system, though there is a difference in quantity. Hence, the surface layer comes to undergo oxidation or changes in properties like the corona discharging system to cause poor image formation and changes in quantity of wear.
Japanese Patent Application Laid-Open No. H11-161120 discloses a method in which, in order to remove discharge products from the photosensitive member surface, the air in the vicinity of the charging assembly is removed by using a suction means such as a fan. Japanese Patent Application Laid-Open No. 2002-296987 also discloses a method in which, about specific substances among the discharge products, a concentration control unit is provided so as to make them into a stated concentration or below. Japanese Patent Application Laid-Open No. 2002-148907 still also discloses a method in which the discharge products are sucked in the direction of the charging assembly.
However, in any of the above background art methods, the discharge products are removed by using an air current generation means such as a fan, and hence the removal of discharge products may come non-uniform depending on the positional relationship with the air current generation means to give the difference in quantity of wear of the surface layer.
Where the discharge products have gathered on one side of the direction of rotational axis, a phenomenon called one-side wear may occur in which the quantity of wear becomes large on one end side of the direction of rotational axis. Such a case may lead up to occurrence of faulty cleaning.